In recent years, use of radio devices using wideband millimeter waves (30 GHz to 300 GHz) has become increasingly widespread. The millimeter-wave radio technology has been expected to be used especially for high-rate radio data communication in the order of gigabit such as radio transmission of high-resolution images (for example, see Non-patent literatures 1, 2 and 3).
However, millimeter waves having high frequencies have a high rectilinear propagation property, and therefore they cause a problem in cases where radio transmission is to be implemented indoors. In addition to the high rectilinear propagation property, millimeter waves are significantly attenuated by a human body or a similar object. Therefore, if a person stands between the transmitter and the receiver in a room or a similar circumstance, no unobstructed view can be obtained, thus making the transmission very difficult (shadowing problem). This problem results from the fact that the propagation environment has been changed because of the increase in the rectilinear propagation property of the radio waves, which results from the increase in the frequency. Therefore, this problem is not limited to the millimeter waveband (30 GHz and above). Although it is impossible to clearly specify the transition frequency at which the propagation environment of the radio waves changes, it has been believed to be around 10 GHz. Note that according to recommendations of the International Telecommunications Union (“Propagation data and prediction methods for the planning of indoor radio communication systems and radio local area networks in the frequency range 900 MHz to 100 GHz,” ITU-R, P.1238-3, April, 2003), a power loss coefficient, which indicates the attenuation amount of a radio wave with respect to the propagation distance, is 22 for 60 GHz in an office, while it is 28 to 32 for 0.9 to 5.2 GHz. Considering that it is 20 in the case of free-space loss, the effects of scattering, diffraction, and the like are considered to be small in higher frequencies such as 60 GHz.
To solve the problem described above, Patent literature 2, for example, discloses a system in which a plurality of transmission paths are provided by installing a plurality of receiving units in the receiver, so that when one of the transmission paths between the transmitter and the receiving units is shielded, the transmission is carried out by another transmission path(s).
Further, as another method for solving the problem, Patent literature 3 discloses a contrivance to secure a plurality of transmission paths by installing reflectors on the walls and ceilings.
The method disclosed in Patent literature 2 cannot carry out transmission when shielding occurs in the vicinity of the transmitter or when all of the installed receiving units are shielded. Meanwhile, the method disclosed in Patent literature 3 requires users to give particular consideration to the configuration. For example, the reflectors need to be installed with consideration given to the positions of the transmitter and the receiver.
However, recent studies on propagation properties of millimeter waves have found out that reflected waves could be utilized without intentionally installing reflectors. FIG. 26 shows a configuration of a system using a wide-angle antenna, and FIG. 27 shows an example of a delay profile of a system using a wide-angle antenna like the one shown in FIG. 26 when the system is used indoors. In the system using the wide-angle antennas shown in FIG. 26, the received power of the dominant wave, which arrives faster than any other waves, is larger than that of any other waves as shown in FIG. 27. After that, although delayed waves such as the second and third waves arrive, their received power is smaller. These second and third waves are waves reflected from the ceiling and the walls. This situation is remarkably different from the propagation environment of radio waves having a lower rectilinear propagation property, such as a 2.4 GHz band used in wireless LANs (Local Area Networks). In 2.4 GHz band, it is very difficult to clearly separate waves in their directions-of-Arrival (DoAs) because of the effects of diffraction and multiple reflections. In contrast to this, in the millimeter waves having a high rectilinear propagation property, although radio waves are relatively clearly distinguished in their DoAs, the number of delayed waves is limited and their received-signal levels are small.
Therefore, when the direct wave is blocked, it is necessary to ensure a sufficient received-signal level by pointing a narrow beam having a high directive gain to a DOA of a reflected wave as shown in FIGS. 25A and 25B in order to continue the transmission by using the reflected wave. However, in order to free users from the need to give particular consideration to the configuration such as the relative positions of the transmitter and receiver, a beam forming technique capable of dynamically controlling a narrow beam is indispensable.
To implement beam forming, it is necessary to use an antenna having function of controlling its directivity. Typical antennas for such use include a phased array antenna. For millimeter waves having a short wavelength (e.g., 5 mm in the case of a frequency of 60 GHz), the phased array antenna can be implemented in a small area, and phase shifter arrays and oscillator arrays for use in those antenna arrays have been developed (for example, see Non-patent literatures 3 and 4). In addition to the phased array antenna, a sector-selectable antenna and a mechanically-direction-adjustable antenna may be also used to implement the antenna directivity control.
Further, as a technique for a different purpose from the beam forming using an antenna array, direction-of-arrival (DOA) estimation techniques have been known. The DoA estimation techniques are used in, for example, radars, sonar, and propagation environment measurements, and used for estimating the DoAs and the power of radio waves to be received at antenna arrays with high accuracy. When a DoA estimation technique is used in propagation environment measurement with an installed radio wave source, an omni (nondirectional) antenna is often used as the radio wave source. For example, Non-patent literature 6 shows an example of such a technique.